diquat

Diquat (diquat dibromide) is a non-selective contact herbicide from the bipyridyl class, widely used in Brazilian agriculture for pre-harvest desiccation (soybeans, beans, potatoes) and control of emerged weeds, as well as applications in non-agricultural areas and control of aquatic macrophytes. It acts rapidly by generating reactive oxygen species (ROS) in the presence of light, causing desiccation and tissue necrosis. In Brazil, it has gained relevance as an alternative to paraquat, which is banned in the country.

Common name: diquat (diquat dibromide)

Number CAS:

Dibromide: 85-00-7

Diquate cation (ion): 2764-72-9

Gross chemical formula: C12H12Br2N2 (dibromide salt). The active form is the diquate cation (C12H12N2²⁺).

Chemical class: Bipyridinium. In the HRAC/WSSA system: Group 22 (electron diversion of Photosystem I). In Brazil (MAPA package inserts): Group D — inhibitors of photosynthesis in Photosystem I.

Main names of commercial products in Brazil: Reglone, Diquat Nortox, Diquat Vanon 200 SL and others.

Development history: Developed in 1955 by RJ Fielden in the dyes division of Imperial Chemical Industries (now Syngenta). The reaction of 2,2'-bipyridine with dibromoethane generated the soluble salt. Its herbicidal properties were identified at Jealott's Hill Research Station by RC Brian and structurally confirmed by RF Homer. Marketed around 1962 as a contact herbicide and desiccant, parallel to paraquat (same period and company). The patent for the active ingredient has expired in all countries.

Mechanism of action: Contact herbicide (predominantly foliar absorption, with some limited xylem translocation). Photosystem I (PSI) electron shift: diquat accepts electrons from ferredoxin, forming a green cationic radical that, upon reacting with O₂, generates ROS (superoxide, hydroxyl radical, etc.). This causes lipid peroxidation of membranes, destruction of chloroplasts and cells, rapid desiccation, and necrosis. The effect is photodependent (requires light for complete activation) and results in rapid death of the affected parts (visible in hours; necrosis in 1–3 days). Little or no residual activity in the soil due to strong adsorption to clays and organic matter.

Control spectrum: Broad spectrum of action against dicotyledonous (broadleaf) weeds and some emerged monocotyledons. Excellent against floating and submerged aquatic macrophytes. Used as a desiccant in crops. Does not control non-emerged plants or plants with insufficient leaf cover; regrowth may occur in perennial plants or those with deep root systems.

Compatibilities and interactions: Compatible with non-ionic adhesive surfactants. Common mixtures with other contact or systemic herbicides for soybean desiccation, improving uniformity without negative synergism on seed physiological quality. May be antagonized or have its efficacy altered by mixtures with products that reduce penetration or with strong oxides/reducing agents. In soil/water: strongly adsorbs to particles, sediments, and plants, limiting leaching and residual activity. In aquatic systems, accelerated degradation by microorganisms after plant death. Always consult specific labels and perform physical and chemical compatibility tests.

Agronomic positioning: Pre-harvest desiccant (soybean R7.3–R8, physiologically mature beans, potatoes — apply 7+ days before harvest) to standardize ripening, facilitate mechanical harvesting, and reduce moisture and impurities in grains. Pre-planting desiccation or burndown in no-till systems (control of emerged weeds). Directed applications in perennial crops (orchards, vineyards) or vegetables. Weed control in non-agricultural areas and aquatic macrophytes.

Other information:

Diquat undergoes little or no metabolism in higher plants and animals; it is predominantly excreted unchanged. Environmental decomposition occurs mainly through photolysis (leaf surfaces) and microbial action in soil and water. This helps to understand its low metabolism and limited environmental persistence under certain conditions. DOI: 10.1021/jf60152a011

The glufosinate + diquat mixture (1 L/ha) provides effective desiccation without negative synergism on the physiological quality of seeds after 8 months of storage (germination, tetrazolium and accelerated aging tests). The combination improves field uniformity without compromising vigor or germination when used for uniformity (not harvest anticipation). DOI: 10.14295/bjs.v2i4.296.